Converting a hexadecimal number to binary in ruby

I am trying to convert a hex value to a binary value (each bit in the hex string should have an equival开发者_JS百科ent four bit binary value). I was advised to use this:

num = "0ff" # (say for eg.)
bin = "%0#{num.size*4}b" % num.hex.to_i

This gives me the correct output 000011111111. I am confused with how this works, especially %0#{num.size*4}b. Could someone help me with this?

You can also do:

num = "0ff"
num.hex.to_s(2).rjust(num.size*4, '0')

You may have already figured out, but, num.size*4 is the number of digits that you want to pad the output up to with 0 because one hexadecimal digit is represented by four (log_2 16 = 4) binary digits.

You'll find the answer in the documentation of Kernel#sprintf (as pointed out by the docs for String#%):

http://www.ruby-doc.org/core/classes/Kernel.html#M001433

This is the most straightforward solution I found to convert from hexadecimal to binary:

['DEADBEEF'].pack('H*').unpack('B*').first # => "11011110101011011011111011101111"

And from binary to hexadecimal:

['11011110101011011011111011101111'].pack('B*').unpack1('H*') # => "deadbeef"

Here you can find more information:

• Array#pack: https://ruby-doc.org/core-2.7.1/Array.html#method-i-pack
• String#unpack1 (similar to unpack): https://ruby-doc.org/core-2.7.1/String.html#method-i-unpack1

This doesn't answer your original question, but I would assume that a lot of people coming here are, instead of looking to turn hexadecimal to actual "0s and 1s" binary output, to decode hexadecimal to a byte string representation (in the spirit of such utilities as hex2bin). As such, here is a good method for doing exactly that:

def hex_to_bin(hex)
  # Prepend a '0' for padding if you don't have an even number of chars
  hex = '0' << hex unless (hex.length % 2) == 0
  hex.scan(/[A-Fa-f0-9]{2}/).inject('') { |encoded, byte| encoded << [byte].pack('H2') }
end

Getting back to hex again is much easier:

def bin_to_hex(bin)
  bin.unpack('H*').first
end

Converting the string of hex digits back to binary is just as easy. Take the hex digits two at a time (since each byte can range from 00 to FF), convert the digits to a character, and join them back together.

def hex_to_bin(s) s.scan(/../).map { |x| x.hex.chr }.join end